1. Field of the Invention
This invention relates to a time indicator and, in particular, to a long term time indicator which provides a rapid and clear indication of expiration.
2. Description of the Related Art
Numerous devices are known which provide, after activation, a visual indication of the passage of a predetermined amount of time. Such a time indicator is useful, for example, as a security badge, as an indicator of the length of time a perishable item has been on the wholesaler's or retailer's shelf and for numerous other uses.
Some known time-indicating devices involve the migration of a colorant, dye or other material through a media. Many of these known time indicators, which are generally short term time indicators, are based on the migration of ink from one substrate through another substrate, i.e., in a path perpendicular to the surface of the substrate. After the ink diffuses for a time period through the substrate(s), it is viewed on a display surface to thereby indicate that the predetermined time has elapsed.
Examples of this diffusion technology can be found in: U.S. Pat. No. 4,212,153 which describes a time indicator where a dye migrates to the surface of an indicator badge; U.S. Pat. Nos. 5,446,705 and 4,903,254 which describe the use of an ink dissolver layer in a time indicator; U.S. Pat. No. 5,058,088 which describes the concept of varying ink dot size and spacing to change the time indication period; U.S. Pat. No. 5,602,804 which describes a time indicator with control of lateral migration; U.S. Pat. Nos. 5,633,835 and 5,822,280 which describe the use of an organic liquid to dissolve a barrier layer and allow for dye migration; U.S. Pat. No. 6,295,252 which describes the use of an accelerator in an adhesive layer; U.S. Pat. No. 6,452,873 which discloses the enablement of dye migration by use of a plasticizer, U.S. Pat. No. 6,514,462 which describes the use of rubber polymers as the diffusion layer in a time-temperature indicator; and U.S. Patent Application Publication No. 2003/0053377 which describes the migration of an amorphous material into a porous matrix when the materials are brought together.
Technologies based on dye diffusion are typically useful for short time intervals such as days or weeks. They are usually not useful for longer time intervals such as months because the color change occurs by gradual dye diffusion which begins the instant the activating adhesive cover is applied over these printed dyes. The time indicator may stay pure white for about a month and then start to gradually change color. During the time interval of gradual color change, the time indicator is in a “gray area” between absolutely YES and absolutely NO. This lack of a sharp transition time is a problem with simple dye diffusion systems.
Other indicators in the prior art rely primarily upon chemical reactions to cause a visually perceptible change over a desired time period rather than merely the migration of fluids or compounds. U.S. Pat. No. 5,045,283 lists various color change reactions that are suitable for time indicator devices. In one example, U.S. Pat. No. 5,045,283 describes the use of acid or base reactant depletion before trigger of an indicator or to control diffusion. U.S. Pat. Nos. 5,085,802 and 5,182,212 also describe the concept of acid or base reactant depletion before trigger of an indicator. U.S. Pat. No. 6,254,969 describes the similar concept of oxygen depletion before trigger of an indicator. U.S. Pat. No. 6,544,925 discloses the use of co-reactants for color formation in a time-temperature indicator system.
The aforedescribed devices are often complicated to adjust for a selected period of time. Adjustments often involve experimentation with many types of chemicals, inks, solvents, etc. to prepare a device which can operate under the conditions expected. Most of the prior art devices gradually change color over a period of time and involve, at best, a guess on how much time has elapsed. When this is combined with the possible variations in temperature, humidity, etc. that may exist in the environment of the time indicator, the viewer may have very little confidence that he is close to the expiration time of the device.
Therefore, there remains a need for a long term time indicator wherein the dye does not begin to appear until the end or near the end of the time interval. Such a time indicator would remain unchanged (white or clear) until near the end of the time interval, and then the color would rapidly or, ideally, instantaneously appear. In essence, what is desirable is a time switch (a color-appearing step-function from white to dark), which stays white until the end of the time interval and then produces a step-function, meaning an instantaneous or rapid color change to clearly show that the time interval has ended. The time indicator would solve the problems with longer term indicators that suffer from an extended “gray time” where there is a slow change in the indication color. The time indicator would allow for a reduced “gray time” for a longer term indicator.